The present invention relates to data communications in a radio communications system, and more specifically, to adaptation of a radio link to a mobile terminal based on current radio communication conditions.
There is an ever increasing convergence of the media industry (including television, video, three dimensional graphics, electronic publishing, and entertainment), the computer industry (including desktop computing, personal computers connected by local area networks, electronic mail, web sites etc.), and the telecommunications industry (both fixed and wireless communications networks). All of these converging industries rely on high-speed data communication capabilities.
High-speed data communication is particularly important for Internet communications. The Internet offers access to an extraordinary variety of information resources across the world. Typically, users make that access from a fixed location, such as their home, business, or school. However, cellular telephones, coupled with an increasing variety of other wireless devices, such as wireless laptops and personal digital assistance (PDAs), are changing otherwise fixed points of access to the Internet to include mobile access by these types of mobile terminals. For the sake of simplicity, the term xe2x80x9cmobile terminalxe2x80x9d is used to encompass all types of wireless devices.
Mobile radio packet data communications employ a different model than the circuit-switched model used, e.g., for traditional mobile radio voice communications. In circuit-switched communications, each communication link is allocated a dedicated radio channel, i.e., a frequency in an FDMA system, a time slot in a TDMA system, or a code in a CDMA system, for the duration of the communication with a mobile radio. Data to other users is not delivered over that dedicated channel, even if there are periods of silence in the communication when no data is being transmitted. Thus, although circuit-switched channels ensure minimal delay and a guaranteed bit rate, which is important for certain applications like voice communications, they are typically underutilized and also are usually limited in bandwidth. That limited bandwidth, while acceptable for certain applications like voice communications, is not well suited for many high-speed data applications that require considerably more bandwidth.
Packet-based data communications are better suited for high-speed data communications. Data packets are delivered individually using a xe2x80x9cbest effort,xe2x80x9d packet-switched network like the Internet. Individual packet routing means that the bandwidth may be used efficiently and that higher bandwidth applications may be accommodated. While wireline data terminals, e.g., personal computers, are capable of utilizing higher, packet-switched network bandwidth, wireless data terminals are at a considerable disadvantage. The bandwidth of the radio interface separating the wireless data terminals from wireline, packet-switched networks like the Internet is limited.
Accordingly, considerable efforts are being made to increase the bandwidth for wireless data communication. That increased bandwidth is particularly important in the radio xe2x80x9cdown linkxe2x80x9d direction from the radio network to the mobile terminal. For example, a mobile terminal user might send in the radio xe2x80x9cup linkxe2x80x9d direction, a low bandwidth request, e.g., a command, to download a web page from a site on the Internet. Downloading the web page and other information (especially graphics) from that web site requires considerably more bandwidth.
Another problem confronting data communications over the radio interface is the variable quality of the radio channel or link from base station to mobile terminal (downlink). The radio channel or link quality depends on a number of factors including the distance between the mobile terminal and a transmitting base station in the radio network, interference from other transmitting base stations and mobile terminals, path loss, shadowing, short term multi-path fading, etc. If the channel quality is good, the base station may modify the signal transmission parameters to increase the data transmission rate from the base station to the mobile terminal. On the other hand, if the channel quality is bad, the signal transmission parameters may need to be adjusted to lower the data transmission rate to ensure that the signal is reliably received.
The process of modifying one or more signal transmission parameters to compensate for channel quality variations is sometimes referred to as xe2x80x9clink adaptation,xe2x80x9d where xe2x80x9clinkxe2x80x9d refers to the radio link between a base station and a mobile terminal. Link adaptation may be accomplished by changing the transmit power of the base station, e.g., increasing the transmit power level for data transmitted to mobile terminals with a bad channel quality. Link adaptation may also be accomplished by changing the type of modulation and amount of channel coding applied to the data to be transmitted by the base station. Moreover, link adaptation may also be performed in the uplink by the mobile terminal.
The present invention is concerned with the latter type of link adaptation because in wide band code division multiple access (WCDMA) communication system, increasing the base station transmit power for an individual subscriber communication adversely impacts other mobile subscriber communications and ultimately limits the overall system capacity. In contrast, changing the modulation and/or coding format to match the current channel quality, without increasing the transmit power, does not adversely impact other mobile subscriber communications or the overall system capacity. For example, mobile terminals close to the base station (presumably experiencing a high channel quality) may be assigned a higher order modulation that provides higher bit rates. A lower order modulation offering a lower bit rate may be employed for communications with mobile terminals farther away from the base station (presumably experiencing lower channel quality). Similarly, when the channel quality is relatively good, higher code rates (i.e., less redundancy) may be employed. Lower code rates (more redundancy) are employed for lower channel quality.
The underlying assumption is that the maximum data rate reasonably supported with each mobile terminal, (i.e., the maximum data rate under the current radio channel conditions to meet a certain performance standard such as a maximum bit error rate), depends on the channel quality experienced by the mobile terminal.
Each base station may be divided into multiple sectors, where each sector serves a particular portion of the geographical area surrounding the base station. For example, each sector of a three sector base station serves approximately one third of the total geographical area surrounding that base station. The mobile terminal estimates the channel quality by measuring the signal quality of pilot signals or other broadcast signals transmitted by nearby xe2x80x9ccandidatexe2x80x9d base station sectors, where some of the sectors may be associated with different base stations. Based on the estimated channel qualities, the mobile terminal determines a maximum data rate at which the mobile terminal can receive data for each base station sector and selects the sector with the highest data rate. The mobile terminal sends a rate/sector request message to one or more base stations in the radio network including information about the current estimated maximum supportable transmission rate as well as the currently requested sector to make the downlink transmission to the mobile terminal. That message also identifies a currently requested base station.
In order to track the rapidly changing conditions of the radio channel, the channel quality measurements are performed at a high rate and the corresponding rate/sector request messages are sent at a high rate in order to track those rapid changes. When a base station receives these rate/sector request messages from several mobile terminals, the base station chooses which sector will handle the data transmission to the mobile terminal based on some scheduling or other decision making algorithm. The base station may, in principle, use any transmission rate below or equal to the rate message from the mobile terminal. However, the following description adopts a decision-making algorithm that results in the highest system throughput. The entire transmission power for high-rate, data packet communications in one sector is assigned to the mobile terminal communication supporting the highest data rate in that sector. The data rate of the other mobile terminal communications in the sector equal is set to zero.
Like the channel quality and associated maximum data transmission rate, the sector requested by the mobile terminal to currently handle the data transmission with the mobile terminal may also vary rapidly. The data packet transmission queue which stores the data packets to be transmitted to the mobile terminal is commonly accessible at a single base station by all sectors of that base station. Accordingly, the data packets from that queue can be quickly routed from one sector to another sector associated with the same base station. On the other hand, the base station currently handling the mobile data transmission cannot be switched as rapidly because a radio network control node is involved in xe2x80x9chanding overxe2x80x9d the mobile data communication (including the data packets in the transmission queue) from the current base station to the new base station.
It is an inefficient use of the limited radio bandwidth to transmit slowly changing information, e.g., a request for a new base station to handle a communication with a mobile terminal, along with rapidly changing information, e.g., requested maximum transmission rate and/or base station sector.
It is also inefficient for a mobile terminal to only request a single sector to transmit data to the mobile terminal at one time. If only one maximum transmission rate/sector request message is transmitted per mobile terminal, and two or more mobile terminals transmit such messages to the same base station requesting the same sector, the base station will choose only one mobile terminal, e.g., the one requesting the highest data rate. Consequently, the transmission of data to the other mobile terminal is delayed.
It is an object of the present invention to provide a link adaptation and sector selection communications scheme that overcomes the inefficiencies noted above.
In accordance with present invention, a mobile terminal sends rapidly changing information relating to the downlink transmission of packet data to a mobile terminal on a fast radio channel. A fast channel provides only a short delay from the time a channel quality measurement is made to the time action is taken in the base station. Moreover, the mobile terminal requests to the base station(s) and the base station action occur frequently in order to track fast channel changes of the channel. This rapidly changing information may include, for example, a current data transmission rate and/or a current base station sector identification. Information that changes more slowly, such as the identification of a base station to handle the mobile terminal communication, is communicated to the base station on a slower radio channel.
At a relatively low reporting rate or when the mobile terminal detects a change in condition that affects base station selection, the mobile terminal requests a new base station to handle the mobile terminal data transmission. The mobile terminal determines the channel quality from candidate sectors associated with the current base station and a corresponding maximum transmission rate for each such sector. A sector with a suitable channel quality/transmission rate is identified by the mobile terminal. Rapidly changing information such as the sector identification and/or maximum data rate information is transmitted to the base station at a relatively high reporting rate. In this way, limited radio bandwidth is not wasted by repeatedly sending information (like the current base station selection) that likely has not changed. On the other hand, rapidly changing information, such as the current channel quality which affects the rate data can be reliably communicated to the mobile terminal, is transmitted at a high frequency so that the base station can rapidly adapt one or more transmission parameters.
The base station receives the current base station identification from the mobile terminal at the lower reporting rate. If there is a change in base station, base station handover procedures may then be initiated. The information received from the mobile terminal at the higher reporting rate is used by the base station to select the particular sector to transmit data to the mobile terminal and/or adjust the current maximum data transmission rate. Since the base station includes a common storage queue of data packets to be sent to the mobile terminal, the switching of sectors associated with the same base station is rapidly executed. The data transmission rate information received from the mobile terminal is used by the selected base station sector to adapt the down link data transmission rate. For example, the amount of channel coding is adapted to accommodate the requested data transmission rate. The type of data modulation may also be adapted to accommodate the requested data transmission rate. As a result of such adaptation, the amount of data transmitted per unit time is increased or decreased in accordance with the current channel conditions.
A mobile terminal preferably makes similar sector/rate requests from plural sectors at the same time. For example, a mobile terminal may identify a first and second sector at the selected base station as potential candidates for transmitting data to the mobile terminal. The mobile terminal determines a first sector data rate and a second sector data rate, and transmits those rates to the base station at the higher frequency. In one non-limiting, example implementation, the first and second sector data rate information can be sent over separate sub-channels which can be either code- or time-multiplexed. When the base station receives the sub-channel request information from the mobile terminal for the first and second sectors, it has the option of selecting a sector suitable for sending the data to the mobile terminal. Although this decision may be determined based solely on the maximum transmission rate information associated with the first and second sectors, (e.g., the highest rate sector is selected), the present invention also permits the base station to make the decision taking into account requests from other mobile terminals. As a result, the base station can accommodate down link transmissions to both mobile terminals using two different sectors. This results in a higher system throughput compared with only one (corresponding to the highest data rate) data transmission being made to one of the mobile terminals.
Along with plural sector/rate requests from each mobile terminal, the present invention utilizes plural antennas at each base station sector to provide additional capacity and flexibility in down link data transmissions. A mobile terminal determines a channel quality or maximum supportable data rate for each of the plural antennas in each candidate sector. The mobile terminal transmits one (and preferably more) messages with a maximum data rate for a selected sector and sector antenna at the higher frequency to the base station. Based on the information received from the mobile terminal, and possibly also from other mobile terminals, one of the plural antennas at a selected sector is used to transmit data at the corresponding rate to the mobile terminal. In effect, the plural sector antennas provide a form of transmit diversity as a xe2x80x9coverlaidxe2x80x9d sectors.